In the extrusion of cellular thermoplastic such as foamed polystyrene and the like, it has been established in the art that in order to obtain satisfactory product from the extrusion line, it is most desirable to accurately control the temperature and flow rate upstream of the extrusion die.
In single screw extruder systems this is quite difficult to accomplish since changing one variable such as temperature will affect the pressure and vice versa. Therefore, as illustrated in U.S. Pat. No. 3,830,901 of Thomas W. Winstead issued Aug. 20, 1974 and entitled "Continuous Process for Extruding Cellular Thermoplastics," a process was developed for achieving desirable foam product from a continuous process utilizing a single screw extruder and a separate cooled metering pump both thermally and physically separated from the extruder but upstream from the extrusion die.
The disclosure and teachings of this patent are incorporated by reference herein.
In providing a metering pump and heat exchange device, which shall hereinafter be referred to as a dynamic cooler apparatus, it has been recognized that because of the high pressures involved and the high viscosities of the plastics used at the temperature levels required for satisfactory foam products to be extruded, there were no suitable commercial devices available for or which could be adapted to this particular application. Accordingly, the need was established for a strong, accurate and efficient dynamic cooler apparatus for use in continuous extrusion processes.
One prior art alternative to dynamic cooler type control is a tandem extrusion system which, however, is not nearly as efficient as dynamic cooler type systems. For example, tandem extrusion systems might produce five pounds per hour per horse power while a dynamic cooler type system can produce at least 15 pounds per hour per horse power. This clearly illustrates the disparity in the efficiencies between dynamic cooler systems using the apparatus of the present invention and those conventional systems known in the art. The inefficiency of these conventional systems is due partly to poor heat transfer, because of thick cross-sections in the extrudate, and partly because of the conveying or pumping inefficiency present in the screw extruders utilized. A typical tandem extrusion system, for example, might utilize a six inch extruder for cooling. Within the flights of the screw conveyor within that extruder, plastic mass is approximately three-quarters of an inch to one inch thick in cross-section and is cooled mostly from the barrel of the extruder by conduction. Since the rate of heat transfer from the plastic is inversely proportional to the thickness of the cross-sections thereof and since a typical plastics extrusion screw requires certain orders of magnitude of the cross-section of the plastic mass which does not lend itself to truly efficient cooling and temperature control; and further, since a typical plastics extrusion screw is not a positive displacement device it is inefficient as a conveyor or pump, it creates a great deal of additional heat in the plastic mass in the process of conveying it along the length of the screw.
Therefore, some form of metering and cooling apparatus is needed which will give both an efficient and controllable thermal transfer for the purposes of cooling the plastic mass as well as accurately and positively displacing that mass from the upstream side of the extrusion die into the extrusion die to thereby provide the desired uniformity of product.
It is therefore an object of the present invention to provide a new and novel dynamic cooler apparatus for efficiently and continuously and uniformly cooling and metering thermoplastic material previously fluxed and mixed with a suitable blowing agent to an extrusion die.
Still another object of the present invention is to provide a new and novel dynamic cooler structure including a positive displacement gear pump in which the gear teeth have mimimum loading imparted thereto while operating at relatively high material pumping pressures and flow rates.
Still another object of the present invention is to provide a new and novel dynamic cooler structure for cooling and metering fluxed and blowing agent mixed thermoplastic material from a screw extruder to an extruding die, which dynamic cooler apparatus achieves the division of the molten plastic into optimally minimum cross-sections and provides a new and novel means for cooling these sections on all four sides thereof.
Yet another object of the present invention is to provide a new and novel dynamic cooler structure including new and novel gear pump means having separate drive means for each gear therein to preclude excessive gear tooth loading.
Yet another object of the present invention is to provide a new and novel dynamic cooler apparatus including a gear-type metering pump in which the shafts are lubricated by the molten plastic being cooled and metered by the apparatus.
These and other objects of the present invention will become more fully apparent with reference to the following specification and drawings which relate to a preferred embodiment of the present invention.